The dihydrochalcone phlorizin (phloretin 2¢-glucoside, synonyms: phlorhizin, phloridzin, phlorrhizin; Fig. 1) is the major phenolic glucoside found in apple trees. Phlorizin has a bitter taste that contributes to the characteristic flavor of cider [1], and its dimerized oxidation products contribute to the color of apple juices [2]. However, subsequent to its isolation from the bark of the apple tree in 1835 [3], phlorizin has attracted most scientific interest through its use as a pharmaceutical and as a tool for human physiology research. Its principal pharmacological action is to produce renal glycosuria and block intestinal glucose adsorption by inhibition of the sodium-linked glucose transporters [4]. Phlorizin and its derivatives have also been shown to be antioxidants in vitro [5], and to have a range of bioactive functions, such as inhibition of lipid peroxidation [6,7], prevention of bone loss [8], enhancement of memory [9] and inhibition of cancer cell growth [10].Until recently, phlorizin was believed to exist only in Malus species. However phloretin glycosides have been reported in the leaves of Australian native sarsaparilla (Smilax glyciphylla) [11], sweet tea (Lithocarpus polystachyus) [12] and at very low levels in strawberry fruit [13]. In apple trees, phlorizin is found primarily in the young shoots, roots, leaves and bark. In fruit, phlorizin is most abundant in the seeds, with intermediate levels in both the core and the skin, and the lowest level in the cortex. Variation has been assessed within apple trees, between orchards, between different The dihydrochalcone phlorizin (phloretin 2¢-glucoside) contributes to the flavor, color and health benefits of apple fruit and processed products. A genomics approach was used to identify the gene MdPGT1 in apple (Malus x domestica) with homology to the UDP-glycosyltransferase 88 family of uridine diphosphate glycosyltransferases that show specificity towards flavonoid substrates. Expressed sequence tags for MdPGT1 were found in all tissues known to produce phlorizin including leaf, flower and fruit. However, the highest expression was measured by quantitative PCR in apple root tissue. The recombinant MdPGT1 enzyme expressed in Escherichia coli glycosylated phloretin in the presence of [3 H]-UDP-glucose, but not other apple antioxidants, including quercetin, naringenin and cyanidin. The product of phloretin and UDP-glucose co-migrated with an authentic phlorizin standard. LC ⁄ MS indicated that MdPGT1 could glycosylate phloretin in the presence of three sugar donors: UDP-glucose, UDP-xylose and UDP-galactose. This is the first report of functional characterization of a UDP-glycosyltransferase that utilizes a dihydrochalcone as its primary substrate.Abbreviations DAFB, days after full bloom; EST, expressed sequence tag; MdPGT1, Malus x domestica UDP-glucose:phloretin glycosyltransferase 1; qPCR, quantitative PCR; UGT, UDP-glycosyltransferase.